1. Field of the Invention
The present invention disclosed herein relates to voltage-controlled oscillators. More particularly, the present invention disclosed herein relates to a voltage-controlled oscillator capable of decreasing phase noise.
2. Discussion of the Related Art
Generally, voltage-controlled oscillators (“VCOs”) are a specific type of electronic circuit which vary control voltages and generate frequency signals. VCOs are widely employed in analog sound synthesizers, mobile communication terminals, and various other similar devices.
FIG. 1 is a circuit diagram of a voltage-controlled oscillator of the prior art. The VCO of FIG. 1 includes an inductor-capacitor (“LC”) bank 110 including inductors L1 and L2, static capacitors Cf1 and Cf2, and variable capacitors Cv1 and Cv2. The static capacitors Cf1 and Cf2 have a static value. A control voltage Vc controls capacitance values of the variable capacitors Cv1 and Cv2 in order to activate first and second oscillation signals VOS1 and VOS2, which are generated from the LC bank 110, in a required frequency. And, a negative resistor circuit 120 functions to maintain the activation of the oscillation signals VOS1 and VOS2.
The VCO shown in FIG. 1 is a differential type of VCO, in which the first and second oscillation signals VOS1 and VOS2 are different in phase by 180°. The negative resistor circuit 120 is connected to a current source circuit 140. A bias circuit 150 supplies a bias voltage to an n-channel metal-oxide-semiconductor (“NMOS”) transistor 141 of the current source circuit 140.
In the VCO of FIG. 1, an amount of noise corresponding to 1/f, wherein f is the frequency of the bias voltage, is generated by the NMOS transistors 141 of the current source circuit 140. The 1/f noise arising from the NMOS transistor 141 is reflected in the first and second oscillation signals VOS1 and VOS2 that are output signals of the VCO.
Therefore, the VCO shown in FIG. 1 has a problem that a rate of phase noise becomes higher due to the 1/f noise.